U.S. patent number 4,514,050 [Application Number 06/400,810] was granted by the patent office on 1985-04-30 for dove prism for convergent light paths.
This patent grant is currently assigned to Bell & Howell Company. Invention is credited to David G. Stites.
United States Patent |
4,514,050 |
Stites |
April 30, 1985 |
Dove prism for convergent light paths
Abstract
An apparatus for correcting aberrations in prisms in convergent
light paths. The prism has a transparent entrance and exit face
which are connected by a hypotenuse surface. The effect of adding a
wedge-shaped addition is made on one of the entrance or exit faces
to create a compensating dispersive spectrum to compensate for
lateral chromatic aberration. The hypotenuse surface is formed as a
spherically convex surface to produce a compensating astigmatic
aberration to compensate for the astigmatic aberration caused by
the entrance and exit faces of the prism.
Inventors: |
Stites; David G. (Elgin,
IL) |
Assignee: |
Bell & Howell Company
(Chicago, IL)
|
Family
ID: |
23585119 |
Appl.
No.: |
06/400,810 |
Filed: |
July 22, 1982 |
Current U.S.
Class: |
359/727; 359/615;
359/833 |
Current CPC
Class: |
G02B
5/04 (20130101); G02B 27/642 (20130101); G02B
27/0025 (20130101) |
Current International
Class: |
G02B
27/00 (20060101); G02B 27/64 (20060101); G02B
5/04 (20060101); G02B 005/04 (); G02B 017/00 () |
Field of
Search: |
;350/444-446,286,168,287 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Corbin; John K.
Assistant Examiner: Sugarman; Scott J.
Attorney, Agent or Firm: Johnson; Neal C. Haggard; Alan H.
Samlan; Alan B.
Claims
What is claimed is:
1. Apparatus for correcting chromatic and astigmatic aberrations in
prisms in convergent light paths comprising:
a transparent entrance face permitting converging imaging rays to
enter the prism;
a transparent exit face permitting the converging imagining rays to
exit the prism;
a third face connecting the entrance and exit faces and reflecting
substantially all of the imaging rays received from the entrance
face internally into the prism and towards the exit face;
the third face disposed at an acute angle with respect to both the
entrance and exit faces, with the angle between the third face and
the entrance face different than the angle between the third face
and the exit face, the angles selected to create a compensating
chromatic aberration to compensate for chromatic abberrations
caused by the prism when used in convergent light paths and the
compensating chromatic aberration is a chromatic spectrum of the
proper magnitude and sign to substantially compensate for lateral
chromatic aberrations of the prism,
the third face further having a spherical, convex curvature which
produces a compensating astigmatic aberration to compensate for the
astigmatic aberration caused by the entrance and exit faces acting
on the converging imaging rays.
2. The apparatus of claim 1 wherein the angle between the third
face and the entrance face is greater than the angle between the
third face and the exit face.
3. The apparatus of claim 2 wherein the difference in acute angles
formed by the third face and the entrance face and the third face
and the exit face is less than one degree.
4. The apparatus of claim 1 wherein the amount of convex curvature
of the third face is in the range of two to six parts per million
of optical path length of prism.
5. The apparatus of claim 1 wherein the third face is a hypotenuse
surface of the prism.
6. A dove prism for use in micro image projection devices which
corrects for lateral chromatic and astigmatic aberrations in
convergent light paths comprising:
a transparent entrance face permitting converging imaging rays to
enter the prism;
a transparent exit face permitting the converging imaging rays to
exit the prism;
a spherical, convex curvature third face connecting the entrance
and exit faces and reflecting the rays received from the entrance
face internally into the prism and towards the exit face;
the curvature of the convex third face selected to produce a
compensating astigmatic aberration to compensate for the astigmatic
aberration caused by the entrance and exit faces acting on the
converging imaging rays;
the third face angularly disposed with respect to the entrance and
exit faces, the angle between the entrance face and the third face
different than the angle between the exit face and the third face,
the angles selected to create a compensating chromatic spectrum of
the proper magnitude and sign to substantially reduce the lateral
chromatic aberration caused by the prism when used in convergent
light paths.
7. The dove prism of claim 6 wherein the third face is a hypotenuse
face of the prism.
Description
This invention relates to dove prisms and more particularly to a
dove prism which is aberration corrected for convergent light to
minimize astigmatism and lateral chromatic aberrations.
In this description of the invention, the dove prism is related to
a very common application, that of image "de-rotation" which is
more correctly identified as "orienting" the image for normal
upright reading in a microfilm reader which projects roll or strip
microfilm images on a viewing screen and/or to a suitable image
copying medium whereby a full size version of the original document
can be recovered. When the microfilm is placed in such a reader for
projecting the image on its viewing screen, it is preferable that
the user be able to view the image in its readable, right side up
orientation. Due to the various orientations of the original
documents when they are photographed in roll microfilm cameras, it
is usually necessary to rotate the image to orient it properly for
the user when the images are projected. Generally, such image
rotation is achieved by the use of a dove prism. The size of the
prism is dependent upon the optical path in the projection
apparatus.
The dove prism has the characteristic of inverting an image from
top to bottom with respect to its base which is horizontal, but not
left to right. Furthermore, the image is rotated twice as fast and
in the same direction as the rotation of the prism. For example, if
the prism is rotated forty five degrees, the image is rotated
through ninety degrees; and if the prism is rotated ninety degrees,
the image is rotated one hundred eighty degrees.
Thus, when the dove prism is used to rotate images for viewing in
their proper orientation, an additional mirror must be employed to
preserve the "handedness" of the projected image so that the image
is not reversed left to right. Alternatively, the microfilm or
object could be inserted in such a manner as to eliminate the
reversal of handedness. Thus, written or text oriented documents
can be rotated for proper reading without being reversed left to
right.
The dove prism does have shortcomings, as will be discussed below,
when used in convergent light paths such as found in microfilm
readers, and in other optical apparatus requiring this type of
prism for image de-rotation, e.g. fire-control periscopes,
telescopes, viewers, etc.
The dove prism is generally manufactured so that it appears as a
truncated isosceles triangle when viewed from a side view
perpendicular to both the entrances and exit faces. In a parallel
light application the imaging rays corresponding to each point on
the object arrive as parallel rays to the hypotenuse face of the
prism. After being refracted downward at the entrance face, the
rays are reflected upward from the hypotenuse and exit after a
second refraction at the exit face where they remain parallel. The
image is inverted without the addition of any substantial amount of
degrading aberrations except those due to fabrication errors in the
three faces and refractive variations in the glass.
The dove prisms in long conjugate, convergent light act as a thick
tilted plate of glass which is tilted with respect to the axis of
the imaging rays. The main problem with using a dove prism in a
convergent light beam or path is that it will introduce a
substantial amount of astigmatism and lateral chromatic aberration.
These image degrading aberrations can impair the image quality as
measured by resolution in microfilm readers and reader printer
applications or other optical instruments requiring such an image
rotation device.
Another problem in using dove prisms for image rotation, is that in
order to minimize or eliminate light bundle vignetting, larger dove
prisms must be utilized especially as the angle of view of the
system is made large. However, the chromatic and astigmatic
aberrations which are increased proportionally when the size of the
prism is increased eventually necessitate some form of aberration
reduction.
Applicant's solution to the problem is to alter the dove prism face
angle between either the entrance face and the hypotenuse face, or
the exit face and the hypotenuse face to establish a thin inclined
glass wedge which produces a compensating chromatic aberration
spectrum. This compensating spectrum is of the proper magnitude and
sign to largely compensate for the lateral chromatic aberration of
the prism. By taking into account the glass refractive index,
thickness, and chromatic dispersion, the correcting angle can be
determined.
Applicant has been able to reduce astigmatism by making the
hypotenuse face slightly convex instead of the normal flat planar
surface which is customary. Thus, an astigmatic aberration of a
compensating nature is also produced by an oblique beam incident on
the internal reflecting face or hypotenuse, thereby offsetting the
astigmatic aberration of the prism.
As the compensating aberrations are established by altering the
prism itself, the correcting factors are built into the prism and
rotate with and as part of the prism. Thus, additional parts which
must be added to the optical system and made to rotate with the
system are not required.
By building the corrective aberrations into the prism itself,
Applicant is able to manufacture the prisms on mass production
automated equipment. This greatly reduces or eliminates hand lapped
or hand ground prisms which are very expensive to make and lack
uniformity and constant quality from prism to prism. The magnitude
of the corrections required are on the order of the manufacturing
tolerances typically needed to make "good quality" prisms. Thus,
this corrective design is partially one of balancing and
controlling the magnitude and sign of the manufacturing tolerances
regarding prism angles and surface shape.
OBJECTS AND ADVANTAGES
Thus, it is an object of the invention to provide a dove prism for
use in convergent light which produces a compensating spectrum to
compensate for the lateral chromatic aberration of the prism. A
related object is to provide a compensated prism which is operable
for all magnifications in a given fixed track length, with some
track length variation being permitted.
Another object is to provide a dove prism which has the hypotenuse
face spherically convex so that an astigmatic aberration of a
compensating nature is produced to overcome the astigmatism of the
prism resulting from its being used in a convergent lightpath.
A related object is to provide a dove prism which has these
compensating aberrations built into the prism so that they rotate
with and are a part of the prism. Associated therewith is to
provide such a prism without the need of supplying additional parts
or components to the optical system.
Yet another object is to be able to use larger dove prisms to
eliminate light bundle vignetting and especially even larger prisms
of lower refractive index while compensating for the otherwise
undesirable increase in lateral chromatic and astigmatic
aberrations.
Yet another object is to provide a prism which produces negligible
image tilt and deviation errors while substantially compensating
for lateral chromatic and astigmatic aberrations.
Still another object is to provide in microfilm reader applications
a prism which allows for the image plane and object plane to be
somewhat non-parallel while still producing an acceptable image
while correcting for aberrations.
Another goal is to be able to manufacture and provide such prisms
at economical costs by designing the prisms so that they can be
mass machined and not hand ground, polished, lapped, or "figured"
one at a time. Such one at a time hand "figuring" is required when
aspheric correction to a reflecting or refracting face is
necessary.
These and other objects and advantages will become apparent upon
reading the description of the drawings and detailed description of
the preferred embodiment.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic illustration of a dove prism of the prior art
in an optical system.
FIG. 2 is an schematic illustration of Applicant's inventive dove
prism which compensates for lateral chromatic aberrations.
FIG. 3 is a schematic illustration of Applicant's inventive dove
prism which compensates for astigmatic aberration.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Turning first to FIG. 1 there is illustrated a schematic
representation of a convergent light path optical system utilizing
a conventional dove prism. Such optical systems are normally found
in apparatus such as microfilm readers and reader printers wherein
image rotation is a desirable or necessary feature. The dove prism
10 inverts the projected image top to bottom but not left to right
in a manner which is commonly known in the art. By rotating the
prism, the image is rotated twice as fast. For example, if the
prism is rotated ninety degrees, the image is rotated one hundred
eighty degrees.
If the dove prism 10 is used in a convergent light beam, as in
microfilm apparatus, it will introduce a substantial amount of
astigmatism and lateral chromatic aberration. These image degrading
aberrations severely limit the resolution in microfilm reader and
reader printer applications and other optical instruments requiring
an image rotation device especially when they combine unfavorably
with similar aberrations caused by the projection lens itself.
Larger dove prisms are desirable and at times necessary in order to
reduce or eliminate light bundle vignetting. However, as the prism
is enlarged, the lateral chromatic and astigmatic aberrations are
increased to such a degree that image degradation results in an
unacceptable image.
In FIG. 1, a typical application would include film 12 lying in a
film plane 14. Generally there would be various images on the film
12. A source of illumination (not illustrated) would pass through
the image on the film 12 through a projection lens 16 which would
cause the illumination or imaging rays to converge at a distant
image plane or viewing screen 18. The imaging rays enter the dove
prism 10 at an entrance face 20. They are then refracted downward
from the entrance face 20 towards a hypotenuse surface 22. The rays
are then reflected upward from the hypotenuse surface 22 and emerge
after a second refraction at an exit face 24. The inverted image is
then focused on the image plane 18.
In general, the index of refraction of optical materials is higher
for short blue wavelengths than for long red wavelengths. This
causes the short wavelengths (illustrated by the solid lines in
FIG. 1) to be more strongly refracted at each surface 20, 24, of
the prism 10 so that the blue light rays form a blue image at 27
while the red light rays (illustrated by the dashed lines) are not
refracted as much and form an image at 25. This results in lateral
color, or chromatic aberration resulting in a displaced set of
images for different wavelengths and is illustrated in FIG. 1 as
the distance K.
Generally, the dove prism 10 has its base angles A and B equal and
often each at forty-five degrees. Other angles can be used
depending upon the desired characteristics of the prism. However,
the dove prism 10 is almost always manufactured such that the
entrance face 20 and exit face 24 are mirror images of each other
with the base angles A and B equal. As the top portion of the prism
is not used, it is truncated leaving a top plane 30.
Turning to FIG. 2, there is illustrated Applicant's inventive
concept to compensate for the lateral chromatic aberration of the
prism. A wedge-shaped element of glass 26 is added to the exit face
24. This creates a new exit face 28 from the prism 10. The angle of
the new exit face 28 with respect to the previous exit face 24 is
indicated by angle C. The angle of the face 28 with the hypotenuse
surface 22 is illustrated as angle D. It can be seen that angle D
will be less than angle B by an amount exactly equal to the amount
that the wedge 26 adds to the old face 24. The additional wedge 26
creates a compensating dispersive spectrum of proper magnitude and
sign to largely compensate for the lateral chromatic aberration of
the prism. The correcting angle C depends on the glass refractive
index and chromatic dispersion. For example, given the following
parameters, we can compute the correcting angle C. The length of
the flat hypotenuse surface 22, H, was approximately 1.8 inches.
The length from the intersection of the hypotenuse surface 22 with
the exit face 24, I, was 32 inches. The distance from the film
plane 14 to the image plane 18, J, was approximately 36 inches. The
index of refraction of the prism n.sub.d, is equal to 1.74 and the
partial reciprocal dispersion V.sub.d is equal to 30. Angles A and
B are 45 degrees. Using these characteristics, the correcting angle
C was found to be approximately 17 minutes. The lateral color, or
chromatic difference of magnification was reduced to less than 5%
of K and is illustrated as L in FIG. 2.
In the actual manufacturing and production of the prism, the
wedge-shaped element 26 was not actually added to an existing
prism. Rather, the prism is made so that the exit face 24 is not
ground, but instead the exit face 28 is the face manufactured and
formed as the exit face of the prism at the required compensating
angle.
FIG. 3 illustrates Applicant's inventive solution to correct for
the astigmatic aberration. By making the hypotenuse surface 22
convex, an astigmatic aberration of a compensating nature is
produced by the obliquely incident imaging rays which offsets the
aberration of the prism. A new convex hypotenuse surface 32 creates
this compensating aberration. In FIG. 3 the amount of the sag or
bow of the convex curvature is greatly exaggerated and is
illustrated as M. The angle between the previous surface 22 and the
convex surface 32 is illustrated as angle F. F is virtually zero
because there is not an angle of error at the vertex at either side
of the prism. In an ideal prism, surface 22 has an infinite
spherical radius, or zero curvature.
The sag or bow M in optical tolerance terms is calculated by the
formula K.lambda./2, whereas .lambda. is equal to the wavelength
and K is the number or fraction of wavelengths or "fringes" of sag
from a plane and typically is equal to 1/2 fringe. Therefore, the
sag M is approximately equal to .lambda./4, or in the order of 5
micro inches. This correction is very sensitive due to internal
reflection in a high refractive index medium.
The convex sag M is dependent upon the ray bundle diameter, the
internal angle of incidence on the hypotenuse face, the refractive
index of the glass of the prism, and the optical thickness or path
length of the prism. Generally, the amount of convex sag M is in
the order of 4 to 12.times.10.sup.-6 inches per 2 inches of optical
path length in the prism. Furthermore, it is sufficient that the
convex curvature be spherical as opposed to other optic forms such
as toric, cylindrical, or other aspheric forms which are sometimes
used as corrective shapes despite their being difficult to
manufacture.
In the production of prisms of this nature, great economic benefits
are achieved if a whole block of prisms are formed at once. This
can be accomplished by placing a group or series of prisms to be
ground in an optical grinding machine. The controls are set for
grinding a spherical surface on hypotenuse faces, and all of the
prisms are ground at once. This improves quality control as the
prisms are not individually hand lapped or ground. It is preferable
to manufacture the prisms, such that they incorporate both the
wedge 26 and the convex hypotenuse surface 32 in one prism such
that the prism compensates for both the lateral chromatic
aberration and astigmatic aberration together.
Such techniques as described above may also be applied to a Schmidt
prism, for example, which can also be used as an image rotator in a
manner similar to a dove prism. This method of compensating for the
lateral chromatic aberration is also suited to any tilted optical
window relative to the optic axis and to other prism forms with
entrance and/or exit faces not normal to the optic axis. The
principal difference being that a prism employs at least one
internally reflecting face to which the light rays are obliquely
incident thereby affording the added opportunity to compensate for
astigmatism caused by oblique refraction at entrance and exit
faces.
The technique is one generally to be recognized as an improvement
to such optical components needing a significant improvement or
extension of suitability to the optical purposes they are put to.
For example, if the simple dove prism can be applied to solve a
problem using only these error correction techniques, this becomes
a valuable method. To invoke the improvements taught herein when
not necessary, would be considered wasteful in effort and
expense.
Thus there has been provided a dove prism that fully satisfies the
objects, aims and advantages set forth above. It is evident that
many alternatives, modifications, and variations will be apparent
to those skilled in the art. Accordingly, it is intended to embrace
all such variations as fall within the spirit and broad scope of
the appended claims.
* * * * *